The epithelial cells lining the small intestine and colon are exposed to many ingested xenobiotics including drugs and environmental carcinogens. Cytochrome P450 isoenzymes, which are encoded by a large superfamily of related genes play an important role in the metabolism of xenobiotics and in the activation of- carcinogens. Preliminary data has indicated that phenobarbital- inducible P450 genes are regulated differently in small intestine and liver and that there is a complex pattern of regulation along the length of the small intestine and within the crypt-villus axis This proposal is based on the hypothesis that expression of genes in the P450 forms expressed in lever, small intestine and colon may determine the response of the individual tissues to toxins or susceptibility to carcinogens. This proposal will address several questions concerning the expression of P450 genes including 1) How are the patterns of expression and inducibility of these genes different in liver, intestine and colon?, 2) Are different members of these highly homologous gene families express in these three tissues?, 3) What are the molecular mechanisms by which the expression of these genes are induced by chemical compounds? and 4) What are the physiologic factors which regulate the expression of P450 genes along the length of small intestine and colon? The major P450 isoenzymes in rat which are inducible by various xenobiotics will be studied in intestinalesinal and colonic mucosa by measuring specific P450 mRNA with cDNA and oligonucleotide probes and P450 apoproteins using immunoblots. The distribution of P450 mRNA within the intestinal cryptvillus axis will be studied using in situ hybridization. P450 from which are expressed in small intestine and colon will be definitively identified using complementary DNA cloning. The mechanism for the induction of P450 expression by chemical inducers will be evaluated by measuring the rate of transcription of the genes and the half-life of mRNA in enterocytes. Finally, physiologic factors such as bile and gastrointestinal hormones, which may be responsible for maintaining expression of P450 genes in the small intestine and colon will be evaluated. These studies have potential importance in several areas. First, differential expression and inducibility of P450 genes in small intestine and colon may have ramifications regarding the response of these tissues to chemical carcinogens this may contribute to the marked difference in the susceptibility of these tissues to the development of cancer. Second differences in the expression of these genes in liver and intestine may provide further insight into the molecular mechanisms which regulate tissue-specific gene expression. And finally, the expression of P450 genes expression along the length of the intestine and as enterocytes mature along the crypt-villus axis.